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MANUAL OF ZOOLOGY. 



the Elasmobranchii, and most Ganoids, the tail is conspicu- 

 ously unsymmetrical (fig. 253, B), and is then said to be 



Fig. 253. A, Sword-fish, showing homocercal tail ; B, Sturgeon, showing the 

 heterocercal form of tail. 



" heterocercal." In these cases, the lower lobe of the tail is 

 conspicuously larger than the upper, owing to the dispropor- 

 tionate development of the haemal rays, and the spinal column 

 is prolonged into the upper lobe of the tail. 



In a recently published and important memoir, Professor 

 A. Agassiz has shown that in Pleuronectes and various other 

 living Bony Fishes, the tail of the early embryo is rounded, 

 and is symmetrically developed at the hinder end of the straight 

 notochord ("leptocardial stage"). Soon the chorda becomes 

 arched upwards, and there appears the first trace of a separa- 

 tion of the tail-fin into two portions, only one of which is 

 destined to remain permanently. The superior of these two 

 divisions; when both have become fully marked out, surrounds 

 the end of the upturned chorda (fig. 254, a), and it must be 

 regarded as an embryonic structure, since it finally disappears. 

 The inferior of the two divisions, on the other hand, is placed 

 below the embryonic tail, and is ultimately developed into the 

 permanent tail. At first the permanent caudal fin has the 

 appearance of a distinct lobe, which looks like a second anal 

 fin. In process of growth, however, the embryonic caudal 

 becomes thrown more and more upwards, and the rays of the 

 permanent caudal acquire a fan -like arrangement. At the 

 stage figured below (fig. 254) the tail is truly "heterocercal," 

 and is wonderfully similar in appearance to the tail of many 

 Palaeozoic Fishes. Finally, however (fig. 255), the turned-up 

 end of the notochord becomes replaced by the long " uro-^ 

 style j " the embryonic caudal diminishes in size and disap- 



